Surface acoustic antenna for submarines

ABSTRACT

The invention relates to surface acoustic antennas for submarines. It consists, in a known antenna, in replacing certain pressure sensors with velocity sensors in order to obtain cardioid directivity with a zero oriented toward the hull of the submarine. It allows the use of baffles serving to attenuate the noise generated inside the submarine.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present Application is based on International Application No.PCT/FR03/00488, filed on Feb. 14, 2003, entitled “SURFACE ACOUSTICANTENNA FOR SUBMARTNES”, which in turn corresponds to FR 02/01940 filedon Feb. 15, 2002, and priority is hereby claimed under 35 USC §119 basedon these applications. Each of these applications are herebyincorporated by reference in their entirety into this application.

FIELD OF THE INVENTION

The present invention relates to acoustic antennas for receivinglow-frequency submarine waves. It relates more particularly to what arecalled surface antennas, the sensors of which are in the form ofpiezoelectric films generally made of PVDF (polyvinylidene fluoride).

BACKGROUND OF THE INVENTION

It is known to place such receiving antennas on the flanks ofsubmarines. Their area is up to several square meters and they arecalled “flank arrays”. In French patent No. 92/06279 filed on May 27,1992, published on Nov. 26, 1992 under No. 2 691 596 and granted on Apr.28, 1995, the Applicant described a flank array composed of severalrectangular panels matching the convex shape of the flank of thesubmarine. Referring to FIG. 2 a of that patent, the panels are mountedon two rails 3, 4 so that the panels are not in contact with the hull, asheet of water separating the panels from the hull. Thus, thetransmission of flexure waves from the hull to the sensor is limited.

However, vibrations and resonances of the hull and of the ancillarystructures of the submarine (especially those emanating from themachinery) continue to pass through the rails.

In addition, since the directivity of the sensors is, in open water,omnidirectional (they are short compared with the central wavelength ofthe listening frequency band), the hull cannot be clad with alow-acoustic-impedance baffle that would improve the acoustic stealth ofthe submarine, since the directivity would then be variable and notcontrollable.

To alleviate these drawbacks, the invention proposes to combine particlevelocity sensors with the pressure sensors so that each receiving panelis directional.

To alleviate these drawbacks, the invention proposes a surface acousticantenna, of the type comprising an array of plane pressure sensors madeof a piezoelectric plastic that are fixed so as to be planar in asupport structure, mainly characterized in that certain of these sensorsare replaced with particle velocity sensors placed in such a way thatthe combination of the signal from the pressure sensors and the signalfrom the velocity sensors is used to obtain a cardioid having a zero forreception normal to one of the faces of the antenna.

SUMMARY OF THE INVENTION

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in the art from the following detaileddescription, wherein the preferred embodiments of the invention areshown and described, simply by way of illustration of the best modecontemplated of carrying out the invention. As will be realized, theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects, allwithout departing from the invention. Accordingly, the drawings anddescription thereof are to be regarded as illustrative in nature, andnot as restrictive.

According to another feature, the particle velocity sensors are formedfrom geophones encapsulated in a mass of syntactic foam, the density ofwhich is the same as that of the encapsulation material forencapsulating all the sensors of the antenna.

According to another feature, the plane pressure sensors are joinedtogether by connection bridges that are curved in the form of a V inorder to form channels for keeping the connection wires for the particlevelocity sensors in place during the operations for molding the antennasystem.

According to another feature, the surface comprising the array ofsensors substantially forms a plane shaped to the surface of the hull ofa carrier ship and the zero of the cardioid is directed toward saidhull.

According to another feature, the carrier ship is a submarine.

According to another feature, the antenna comprises at least one panelconsisting of fifteen pressure sensors and six velocity sensorsregularly interspersed among these pressure sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become clearlyapparent from the description that follows, with regard to the appendedfigures which represent:

FIG. 1, a plane sectional view of the panels of an antenna according tothe prior art;

-   -   FIG. 2, a view under the same conditions of an antenna according        to the invention;    -   FIG. 3, a perspective view of a geophone used in an antenna        according to the invention; and    -   FIGS. 4 and 5, perspective views of a panel according to the        invention before and after molding.

DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the invention, a surface acoustic antenna for submarines asdescribed in French patent No. 2 691 596 is essentially characterized inthat each panel comprises particle velocity sensors whose sensitivityaxis is normal to the plane of the panel and in that the correspondingcenter of phase is coincident with the center of phase of the pressuresensors, thus making it possible to obtain cardioid directivity.

FIG. 1 shows a view of the pressure sensors located within a panelaccording to the prior art and it corresponds to FIG. 5 of French patentNo. 2 691 596. It will be recalled that the pressure sensor has abimorph structure, that is to say it is formed from two layers of PVDFpiezoelectric film separated by a central electrode forming the hotspot. The two layers are covered with two outer electrodes that areelectrically connected to form the cold spot.

Thus, the view shown in FIG. 1 is a plane cross section through thepanel level with the central or outer electrodes. Each electrode isformed from a set of square plates 10 joined together by narrow bridges11. The two connections 12 for the output signal are located at one endof the array.

FIG. 2 shows a view of the modified panel according to the invention, inthe same cross section as that of FIG. 1. Velocity sensors 20 are placedat selected locations in order to bring into coincidence the two centersof phase corresponding to the two signals measuring the pressure and thevelocity component normal to the panel. This result is obtainedgeometrically using the symmetry of the locations where the measurementsare made.

In the embodiment example shown in FIG. 2, six of the twenty-onepressure sensors 20 have been removed and six velocity sensors 20 havebeen placed at the center of the spaces thus left.

According to a preferred embodiment, shown schematically in FIG. 3, thevelocity sensors are geophones 301 encapsulated in a syntactic foam 302having the same density as the encapsulating polyurethane in which thepressure sensors are molded, as described in French patent 2 691 596.

The series cabling of the geophones is indicated in FIG. 2 and has noparticular features except that the bridges 21 are V-shaped so as toform a channel for reducing the movement of the cables during themolding operation, as may also be clearly seen in FIG. 4.

Coming into the connector 12 are two wires for the omnidirectionalpressure signal output by the panels 10 and two wires for the velocitysignal output by the sensors 20. The directivity of the signal from thesensors 20 is as cos?, where ? is the angle of incidence of the waverelative to the sensitivity axis of the sensor. As is widely known, theaddition of these two signals provides a signal whose directivity is inthe form of a cardioid, with the “zero” direction normal to the paneland oriented rearward, and therefore toward the hull.

This thus results in strong rejection of the waves coming from the rearand in hydrophone sensitivity independent of the support to which theantenna is fixed. This support may therefore be a matched baffle.Experiments have shown that it is possible to achieve a gain of around10 dB in terms of rejection of noise specifically of mechanical origin.

FIG. 4 shows, in a perspective view, a panel according to the inventionbefore molding and FIG. 5 shows such a panel after molding, after asuitable polyurethane material 50 has been poured in, the level of whichcomes flush with the upper part of the geophones. To complete the panel,a layer of neoprene having a composition identical to that of thesurround 40 is cast on top.

The panel thus obtained can be installed directly on the hull of thesubmarine, or else on a material with a low acoustic impedance depositedon the hull, which thus improves the acoustic stealth of the submarine.

Without departing from the scope of the invention, the geophones may bereplaced with accelerometers or any other directional sensor.

It will be readily seen by one of ordinary skill in the art that thepresent invention fulfills all of the objects set forth above. Afterreading the foregoing specification, one of ordinary skill will be ableto affect various changes, substitutions of equivalents and variousother aspects of the invention as broadly disclosed herein. It istherefore intended that the protection granted hereon be limited only bythe definition contained in the appended claims and equivalents thereof.

1. A surface acoustic antenna placed on a flank of a carrier ship havinga hull, comprising: an array of plane pressure sensors made of apiezoelectric plastic that are fixed so as to be planar in a supportstructure, wherein certain of these sensors are particle velocitysensors placed so that the center of phase of the velocity sensorscoincides with that of the pressure sensors, the signal output by thepressure sensors and that output by the velocity sensors being combinedso as to achieve rejection of the acoustic waves via that face of theantenna facing the hull of the carrier ship.
 2. The antenna as claimedin claim 1, wherein the particle velocity sensors are formed fromgeophones encapsulated in a mass of syntactic foam, the density of whichis the same as that of the encapsulation material for encapsulating allthe sensors of the antenna.
 3. The antenna as claimed in claim 1,wherein the plane pressure sensors are joined together by connectionbridges that are curved in the form of a V in order to form channels forkeeping the connection wires for the particle velocity sensors in placeduring the operations for molding the antenna system.
 4. The antenna asclaimed in claim 1, wherein the surface comprising the array of sensorssubstantially forms a plane shaped to the surface of the hull of acarrier ship, the combination of pressure sensors and velocity sensorsforming a cardioid shaved diagram with zero a reception directed towardthe hull of the carrier ship.
 5. The antenna as claimed in claim 4,wherein the carrier ship is a submarine.
 6. The antenna as claimed inclaim 1, comprising at least one panel consisting of fifteen pressuresensors and six velocity sensors regularly interspersed among thesepressure sensors.
 7. The antenna as claimed in claim 2, wherein theplane pressure sensors are joined together by connection bridges thatare curved in the form of a V in order to form channels for keeping theconnection wires for the particle velocity sensors in place during theoperations for molding the antenna system.
 8. The antenna as claimed inclaim 2, wherein the surface comprising the array of sensorssubstantially forms a plane shaped to the surface of the hull of acarrier ship, the combination of pressure sensors and velocity sensorsforming a cardioid shaped diagram wit a zero reception directed towardsaid hull.
 9. The antenna as claimed in claim 3, wherein the surfacecomprising the array of sensors substantially forms a plane shaped tothe surface of the hull of a carrier ship, the combination of pressuresensors and velocity sensors forming a cardioid shaped diagram wit azero reception directed toward said hull.
 10. The antenna as claimed inclaim 2, comprising at least one panel consisting of fifteen pressuresensors and six velocity sensors regularly interspersed among thesepressure sensors.
 11. The antenna as claimed in claim 3, comprising atleast one panel consisting of fifteen pressure sensors and six velocitysensors regularly interspersed among these pressure sensors.
 12. Theantenna as claimed in claim 4, comprising at least one panel consistingof fifteen pressure sensors and six velocity sensors regularlyinterspersed among these pressure sensors.
 13. The antenna as claimed inclaim 5, comprising at least one panel consisting of fifteen pressuresensors and six velocity sensors regularly interspersed among thesepressure sensors.
 14. A retrofit kit for a purpose acoustic antennaplaced on a flunk of a carrier ship having a hull, said antennaincluding an array of plane sensors made of a piezoelectric plastic thatare fixed so as to be planar in a support structure said retrofit kitcomprising one or more plastic velocity sensors to replace some of saidarray of plane sensors placed so that the center of phase of thevelocity sensors coincides with that of the pressure sensors, the signaloutput by the pressure sensors and that output by the velocity sensorsbeing combined so as to achieve rejection of the acoustic waves via tatface of the antenna facing the hull of the carrier ship.
 15. The antennaas claimed in claim 14, wherein the particle velocity sensors are formedfrom geophones encapsulated in a mass of syntactic foam, the density ofwhich is the same as that of the encapsulation material forencapsulating all the sensors of the antenna.
 16. The antenna as claimedin claim 14, wherein the surface comprising the array of sensorssubstantially forms a plane shaped to the surface of the hull of acarrier ship, the combination of pressure sensors forming a cardioidshaped diagram with a zero reception directed toward said hull.
 17. Theantenna as claimed in claim 14, wherein the cater ship is a submarine.18. The antenna as claimed in claim 14, a comprising at least one panelconsisting of fifteen pressure sensors and six velocity sensorsregularly interspersed among these pressure sensors.
 19. The antenna asclaimed in claim 14, wherein the particle velocity sensors are formedfrom geophones encapsulated in a mass of syntactic foam, the density ofwhich is the same as that of the encapsulation material forencapsulating all the sensors of the antenna.